Bumping gaia.json for 1 gaia-central revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/8255b237e446
Author: Dave Huseby <dhuseby@mozilla.com>
Desc: Bug 915672 - Modify marionette test driver scripts to add support for gathering power draw data. r=dhunt r=jgriffin